In the course of collecting population samples of Mniaceae in southwestern China, a new species was discovered. Plagiomnium jiuzhaigouense occurred in large patches on spruce logs in forests dominated by spruce, fir and larch intermixed with deciduous hardwoods such as maples and birches at elevations between 2,500 m and 3,000 m. It appears to be most similar to P. tezukae, a Southeast Asian endemic known from Japan, Korea, and China. The new species differs in having smaller leaves, shorter leaf cells, and longer teeth composed of two or three cells. Based on sequencing of two regions of chloroplast DNA (rps4 and rpl16) and one of nuclear DNA (ITS2), P. jiuzhaigouense is placed between Section Rosulata and Section Undulata, rather than in Section Rosulata to which P. tezukae belongs. This result presents a challenge to the traditional delimitation of sections in the genus.